Yeah, you really have to stack the odds against royals to make em break, and make sure to have a unit of super fast light cavalry ready to pick him off as he runs. Theeir horses seem to pick up an adrenaline rush as soon as the rout/withdraw begins.

On the vices related to killing prisoner, how do you achieve "99 “Butcher”
“He is famous for the massacre of thousands of prisoners in a single battle. Few can match the horror of this deed. +2 Dread”
It doesnt seem to follow on from scant mercy, merciless, blood lover ones.
Maybe fight a huge battle and capture the suggested thousands?